The present invention relates to vehicle anti-lock braking systems, and more particularly to a system and method for determining anti-lock braking slip and deceleration thresholds.
Motor vehicle anti-lock braking systems modulate hydraulic brake pressures upon detection of insipient wheel lock to maximize friction between tires of a vehicle and a road surface. At incipient wheel lock, the brake pressures are initially reduced by an amount that is based on an assumed coefficient of friction between the tires of the vehicle and the road surface. The brake pressures are re-applied once the acceleration of the wheels exceed a predetermined acceleration value.
Since the coefficient of friction is ordinarily unknown, the initial brake pressure reduction is typically calculated for a worst-case road surface, such as glare ice. This approach provides a somewhat degraded braking performance when the road surface has a relatively high coefficient of friction. Most road surfaces have substantially higher coefficients of friction than glare ice. In other words, the brake pressures are reduced more than is typically required by the road surface.
After the initial brake pressure reduction, the coefficient of friction is estimated based on a time period that is required for the wheels to accelerate to the reference acceleration value. The rate of brake pressure re-application is determined based on the estimated coefficient of friction. Although the coefficient of friction can be initially estimated from the brake pressure or the brake pedal force, the sensors for obtaining such information significantly increase the cost of the anti-lock braking system. The estimated coefficient of friction is subject to errors when the relationship between brake pressure and brake torque deviates from the norm.
Some conventional anti-lock braking systems include a xe2x80x9cpeak seekingxe2x80x9d control method that slowly adjusts the wheel slip and wheel deceleration thresholds by applying rate controlled brake pressure increases. This peak seeking method may require several apply and release cycles to find the correct slip and deceleration target thresholds. Time wasted during the peak seeking control method lengthens the total stopping distance on all surface types.
A control system and method according to the present invention operates a vehicle anti-lock braking system. The braking system includes a brake pedal and a brake modulator. The brake system reduces braking pressure by an initial pressure reduction after detecting insipient wheel lock. Vehicle deceleration is measured as a function of brake pedal position. A first table is updated with the vehicle deceleration and the brake pedal position. A coefficient of friction of a road surface is estimated based on the first table. Wheel slip and deceleration target thresholds are determined based on the coefficient of friction.
In other features, the wheel slip and deceleration target thresholds are used to populate a command table for the anti-lock braking system. Brake heat, brake torque, vehicle weight and grade are estimated.
In still other features, a wheel recovery timer is set equal to zero. Onset vehicle speed is determined. An actual recovery time that is required for wheels of the vehicle to accelerate to a predetermined acceleration level is timed. An expected recovery time is calculated and compared to the actual recovery time.
In yet other features, a recovery delta is calculated based on the expected and actual recovery times. The coefficient of friction is updated if the recovery delta is greater than a first constant. An adjusted coefficient of friction is calculated based on the recovery delta and the coefficient of friction. Wheel slip is calculated based on the adjusted coefficient of friction, the coefficient of friction, and the target wheel slip. Wheel deceleration is calculated based on the adjusted coefficient of friction, the coefficient of friction, and the target wheel deceleration. The command table is updated with the wheel slip and deceleration. Release and apply pressures are calculated from the adjusted coefficient of friction, the coefficient of friction and the grade.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.